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Re: a finicky rule

  • To: mathgroup at smc.vnet.net
  • Subject: [mg41734] Re: [mg41714] a finicky rule
  • From: Bobby Treat <drmajorbob+MathGroup3528 at mailblocks.com>
  • Date: Tue, 3 Jun 2003 07:13:16 -0400 (EDT)
  • Sender: owner-wri-mathgroup at wolfram.com

Look at the output of

Trace[x^3 - x^2 - 1 /. pullx ]

It appears that -1 is matching x in the pullx pattern.

Bobby

-----Original Message-----
From: Selwyn Hollis <selwynh at earthlink.net>
To: mathgroup at smc.vnet.net
Subject: [mg41734] [mg41714] a finicky rule

Group,

I'm having a difficult time understanding the following behavior.

I define this rule:

    pullx := c_. + (a_.) x_^(p_.) + (b_.)x_^(q_.) ->
              c + x (a x^(p-1) + b x^(q-1))

and it works here:

    x^3 + x^2 - 1 /. pullx

        -1 + x*(x + x^2)

but not here:

    x^3 - x^2 - 1 /. pullx

        -1 - x^2 + x^3

even though the pattern matches:

    MatchQ[x^3 - x^2 - 1, pullx[[1]]]

        True

I thought perhaps some simplification was being done on the result, but:

    (x^2 - x)*x - 1

        -1 + x (-x + x^2)

Any ideas? (This is 4.1, Mac OS X.)

Thanks.

-----
Selwyn Hollis
http://www.math.armstrong.edu/faculty/hollis


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